1) Field of the Invention
The field of the present invention relates to removable cartridge disk drives in general and, more particularly, to a method and system of encoding and decoding servo information on a removable disk cartridge.
2) Background of the Related Art
Removable cartridge disk drive systems have been available on the market for some time. Like fixed disk drives, removable cartridge disk drive systems provide large storage capacities with relatively rapid access times at low cost. However, unlike fixed disk drives, removable cartridge disk drive systems enable a user to easily replace a relatively high capacity disk, allowing for convenient exchange of large amounts of information between remote sites and for greatly increased system storage capacity.
Like fixed magnetic disk drives, removable cartridge magnetic disk drive systems use a magnetic read/write head to read and write data stored as magnetic fields on a magnetic disk surface. The disk surface is divided into a number of concentrically arranged tracks where customer data records are stored. To read or write data to the disk, the disk surface is rapidly rotated and the read/write head passes over its surface, following the circumferential path of the track or tracks where the data are to be stored or read.
For proper data storage and recovery, the read/write head must locate each track wherein data are to be written or read, and follow its path accurately along the disk surface. Thus, the disk drive requires some means of identifying tracks and of controlling the precise radial position of the read/write head so that read/write operations are performed on the desired track. Accurate read/write head location is particularly important during write operations to prevent the head from inadvertently writing over previously written data which the user wishes to keep. To accomplish this, servo position information is pre-recorded on the magnetic disk at time of manufacture, and is used by a servo system to control head motion when seeking between tracks, and to regulate head position on track when data are being written or reproduced. The servo information is preferably recorded at multiple circumferential locations, at uniform intervals on every track of the disk. Servo sections on a given track are radially contiguous with their counterparts on the adjacent tracks.
Many types of servo systems have been devised to detect and correct the alignment between the read/write head and the centerline of a track, and to provide position information to the read/write head identifying the particular track being followed. An additional component of most prior art servo patterns is an encoded bit representing track start, or index point. Aspects of some of these systems are described in:
B. McKnight, "A Track Locating Servo System Utilizing the Data Heads as Absolute Position Transducers," IEEE Tr. Mag., Vol. Mag-14, July 1978; U.S. Pat. No. 5,255,131, entitled "Asynchronous Servo Identification/Address Mark Detection for PRML Disc Drive System;" U.S. Pat. No. 4,933,786, entitled "Fault-Tolerant Index Patterns and Decoding Means;" and U.S. Pat. No. 4,459,232, entitled "Phase Modulated Servo System."
Customer data records are recorded in the space between servo sectors. During formatting of these data, a header is initially written for each record, by the data path controller, to provide full track address and circumferential position information describing the location of the associated record. The header is used subsequently to locate and to confirm record identity prior to writing or reading the associated record. In toned-recording systems, the header may also contain information identifying the locations where data records are split around servo bursts. The header is used solely by the data path controller and is not used by the servo system.
Removable disk cartridge technology continues to advance, placing additional requirements on servo system. Track and bit area densities on magnetic disks continue to rise, yielding removable disk cartridges with the potential for greater storage capacity. To realize higher data storage capacities, servo sample rates must rise in order to achieve improved tracking at higher track densities, leading to a need for more compact, efficient servo patterns. Nevertheless, accurate read/write head position information is required in both radial and circumferential directions to prevent inadvertently writing over data previously stored on the disk.
With increased servo sector densities, and in presence of mechanical tolerances, it is possible that significant circumferential skew can exist between servo sectors served by different heads; this skew can in extreme cases exceed one servo sample interval. It is nonetheless necessary when changing heads to have an immediate indication of track position on the new head, without having to wait up to an entire disk revolution to read an index mark.
Moreover, increasing area densities leads to increasing susceptibility to servo errors due to flaws or imperfections in the recording medium. Therefore there is a need for a headerless servo sector format which can provide robust track position and circumferential orientation information to a sample-data servo system.
Another problem arises in removable cartridge disk drive systems which use rotary activators in conjunction with magneto-resistive read/write heads having separated head and write gaps. In such systems, the head may require radial repositioning between read and write operations, precluding accurate recovery prior to write of the conventionally-used header field. It is therefore desirable to dispense with the header. Omission of headers also makes more space available for storage of customer data. Consequently, it is desired to provide means, other than use of headers, of identifying the circumferential orientation of the read/write head within any given track, especially if the system has a high potential inter-head circumferential skew, and is using so-called headerless formats.
Accordingly, it would be advantageous to provide a more compact servo pattern which also supplies an absolute indication of circumferential orientation of a read/write head within a track without the use of an additional dedicated field within the servo sector to contain this circumferential information. It would further be advantageous to provide a servo pattern which is robust in the presence of flaws and imperfections in the magnetic recording medium. It would further be advantageous to provide a servo pattern comprising servo sectors which provide an indication of circumferential orientation of a read/write head to allow a sample-data servo system to instantly regain sync during head-switching operations, in presence of head skew in excess of one servo sample interval in length. Other and further objects and advantages will appear hereinafter.